Modular circuit board

ABSTRACT

Prior power converters have utilized discrete components mounted on circuit boards of differing designs. Failure of one or more of the components requires testing each component to determine which is faulty. The faulty component must then be replaced. This results in significant down time for the inverter and can require stocking a large number of specialized boards. In order to overcome these problems, an inverter is assembled using modular circuit boards. Each circuit board includes components mounted on a multilayer substrate and a heat exchanger which cools high power components. When a malfunction of a component occurs, the entire board containing the failed component may be readily replaced so that inverter down time is minimized. The use of modular circuit boards also reduces stocking requirements and inventory costs.

TECHNICAL FIELD

The present invention relates generally to power converters, and moreparticularly to a modular substrate which is readily configured toprovide neutral, negative, or positive circuit boards. These circuitboards may be electrically connected to provide positive, negative, andneutral inverter switches which may be used in one leg of a three phaseneutral point clamped inverter.

RELATED TECHNOLOGY

In a variable-speed, constant-frequency (VSCF) power generating system,variable-frequency power produced by a brushless, synchronous generatormay be converted by a power converter into constant-frequency AC power.The power converter includes a rectifier bridge to convert thevariable-frequency power into DC power on DC link conductors and aninverter to convert the DC power into the constant-frequency AC power.The inverter may be, for example, of the neutral point clamped typehaving positive and negative switch assemblies connected in seriesacross the DC link conductors and a neutral switch assembly connectedbetween a neutral voltage and a junction between the positive andnegative switch assemblies.

In general, each of the positive and negative switch assemblies includesdriver and driven transistors connected together in a Darlingtonconfiguration, base biasing components, snubber components, and aflyback diode connected across the driven transistors.

The neutral switch assembly includes all of these components with theexception of a flyback diode connected across the driven transistors.The neutral switch assembly also includes four high power diodesconnected in a bridge configuration which permit bi-directional currentflow between the neutral voltage and the phase output. The neutralswitch assembly thus includes three additional power diodes compared toeither the positive or negative switch assemblies.

The components of each switch assembly handle large magnitudes ofcurrent and are subject to failure. A component failure may cause outputcurrent harmonic content to increase beyond acceptable limits or mayrender the inverter inoperative. When components of a switch fail, it isnecessary to shut down the inverter in order to isolate and replace thefailed component. The processes of isolating and replacing the failedcomponent may be lengthy; leading to significant down time for theinverter.

The repair process may be shortened considerably by packaging each ofthe switch assemblies as a separate unit on a single printed circuitboard. Thus, for each inverter leg, only three components, (i.e. eachswitch assembly) need be tested in order to isolate a failed component.Once the failure is isolated to a single board, the entire board may bereplaced and the failed components on a faulty board repaired"off-line".

Replacement of a complete board minimizes the time that an inverter isout of service, although at least two different sets of boards are used,one for the positive and negative switch assemblies and one for theneutral switch assembly.

SUMMARY OF THE INVENTION

In accordance with the present invention, a constant-frequency AC outputapparatus includes at least two modular substrates arranged in a stackedconfiguration, means for mounting electrical components on each modularsubstrate to provide at least two circuit boards, and means forconfiguring the circuit boards to produce an alternating current outputfrom a direct current input.

Preferably, a constant-frequency AC output apparatus of this inventionincludes a modular substrate, where the substrate includes a mainelectrically conductive bus layer, first and second layers ofelectrically insulating material disposed on first and second sides ofthe main layer, a series of traces disposed on the first layer ofelectrically insulating material, a third layer of electricallyinsulating material disposed on the series of traces, a planar resistorlayer disposed on the second layer of the electrically insulatingmaterial, and a fourth layer of electrically insulating materialdisposed on the planar resistor. The most preferred modular substrateincludes an electrically conductive heat exchanger disposed on thefourth insulating layer.

An alternating current output apparatus of this invention includes twocircuit boards that are configured to provide a positive and a negativeswitch assembly where both circuit boards contain the same number ofcomponents for a positive switch assembly and for a negative switchassembly.

Preferably, an alternating current output apparatus includes threecircuit boards configured to provide a positive, a negative, and aneutral switch assembly in which the negative and the positive circuitboards contain components for the neutral switch assembly as well ascomponents for either a positive or a negative switch assembly. In thispreferred embodiment, the apparatus includes circuit boards for thenegative, positive and neutral switch assemblies with three power diodesand three power transistors mounted on each circuit board.

A faulty inverter due to failure of a component of a particular switchassembly may be easily repaired by substituting a similarly configuredboard for the board containing the failed component. In addition, thepositive, negative and neutral circuit boards are made using identical,modular substrates, there is no need to manufacture and stock differentsubstrates for each type of switch assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 comprises a block diagram of a variable-speed, constant frequencypower generating system in conjunction with a prime mover;

FIG. 2 comprises a block diagram of the inverter shown in FIG. 1;

FIG. 3 comprises a block diagram of one leg of the inverter shown inFIG. 2;

FIG. 4 comprises a schematic diagram of power switches together withbase biasing, snubber and flyback circuitry of the inverter leg shown inblock diagram form in FIG. 3;

FIG. 5 is a perspective view of a positive circuit board configured toform a positive switch assembly and a portion of the neutral switchassembly;

FIG. 6 is a perspective view of a second side of the circuit board ofFIG. 5;

FIG. 7 comprises an exploded perspective view of the circuit board ofFIG. 5 illustrating various electrical components separated from thecircuit board;

FIG. 8 comprises an exploded perspective view from the first side of thecircuit board of FIG. 5 illustrating several of the layers thereof;

FIG. 9 comprises an exploded perspective view from the second side ofthe circuit board of FIG. 5 illustrating the remainder of the layersthereof;

FIG. 10 comprises a plan view of the first side of the circuit board ofFIG. 5;

FIG. 11 comprises a plan view of the first side of a negative circuitboard configured to form a negative switch assembly and a portion of aneutral switch assembly; and

FIG. 12 comprises a plan view of the first side of a neutral circuitboard configured to form a portion of a neutral switch assembly.

DESCRIPTION OF THE PREFERRED EMBODIMENT

This invention encompasses a constant frequency alternating currentapparatus which includes components mounted on a substrate having a mainlayer including an electrically conductive portion and an electricallynon-conductive portion adjacent thereto, the substrate having first andsecond opposite faces wherein the electrically conductive andnon-conductive portions substantially span the entire distance from thefirst face to the second face. A layer of insulating material isdisposed on one side of the first and second faces and an electricallyconductive component, such as a conductive trace, is disposed on thelayer of insulating material.

The substrate also includes via holes which extend through the layer ofinsulating material for coupling the electrically conductive componentto the electrically conductive portion of the substrate.

Specifically, the substrate layer includes a plurality of spaced coppersections separated by an electrically non-conductive filler material.Electrically insulating layers are deposited on first and second sidesof the main layer. One or more planar etched foil resistors are disposedon one of the insulating layers and a further electrically insulatinglayer is deposited on the resistors. An electrically conductive heatexchanger is disposed on the last-named insulating layer.

Disposed on the other insulating layer is a series of traces comprisingprinted strips of electrically conductive material. Another insulatinglayer is deposited on the trace layer. Via holes and apertures areformed in the various layers to allow connection to various components.The assembled components described above form a modular substrate andadditional components may be mounted on the blank substrate to formpositive, negative and neutral circuit boards.

The design of the substrate permits the positive, negative, and neutralcircuit boards to be stacked next to one another so that the size of theresulting inverter is minimized. Cooling fluid is provided to the heatexchangers to effectively remove excess heat from components located inthe inner board or boards intermediate the ends of the stack. Thus, alarge number of heat-producing components may be assembled within asmall space without being damaged by excess heat build-up.

Referring now to FIG. 1, a variable-speed, constant-frequency (VSCF)power generating system 10 converts variable-speed motive power producedby a prime mover 12, such as an aircraft jet engine, intoconstant-frequency AC electrical power on a load bus 14. It should benoted that various contactors which connect the VSCF system 10 to the ACload bus 14 are not shown for the sake of simplicity.

The VSCF system 10 includes a brushless, synchronous generator 16 whichconverts the variable-speed motive power produced by the prime mover 12into variable-frequency AC power. This AC power is converted by an AC/DCconverter and transient suppressor 18 into DC power on a DC link 20including DC link conductors 20a, 20b. A DC link filter (not shown) maybe provided to minimize ripple in the DC power on the DC link 20. Theinverter 22 converts the DC power into constant-frequency AC power whichis filtered by an optional filter 24 and provided to the AC load bus 14.

Referring now to FIG. 2, the inverter 22 is preferably, although notnecessarily, of the three-phase type including first through thirdinverter legs 30a, 30b and 30c which are coupled to the DC linkconductors 20a, 20b and a neutral terminal 32 at which a neutral voltageis produced. The neutral terminal comprises the junction between firstand second capacitors C1 and C2 coupled across the DC link conductors20a, 20b.

FIG. 3 illustrates the inverter leg 30a in greater detail, it beingunderstood that the legs 30b and 30c are identical thereto. Positive andnegative switch assemblies 34 and 36 are coupled in series across the DClink conductors 20a, 20b. A phase output is produced at a terminal 38which comprises the junction between the switch assemblies 34 and 36. Aneutral switch assembly 40 is coupled between the phase output terminal38 and the neutral terminal 32.

The switch assemblies 34, 36, 40 are operated to produce an AC waveformat the output terminal 38 comprising a series of alternating positiveand negative half-cycles. The positive half-cycle is produced byalternately operating the switch assemblies 34 and 40 in a pulse-widthmodulated (PWM) mode. Each negative half-cycle is produced byalternately operating the switch assemblies 36 and 40 in a PWM mode. Theoutput voltage produced at the terminal 38 thus switches between apositive or negative level and the neutral voltage, but not between thepositive and negative levels. The inverter is thus referred to as aneutral point clamped type inverter. It should be noted that inverter 22need not be of this type, in which case the neutral switch assembly 40of the leg 30a and the corresponding neutral switch assemblies of thelegs 30b and 30c would not be needed. In this case, the switchassemblies 34 and 36 may be alternately operated in a PWM mode or may beoperated to produce a stepped output waveform, if desired.

Referring now to FIG. 4, the positive switch assembly 34 includes adriver transistor QD+ which is connected in a Darlington configurationwith driven transistors Q1+ and Q2+. Connected to the bases of thetransistors QD+, Q1+ and Q2+ is base biasing circuitry including diodesD1+ through D3+ and DB1+ through DB3+ together with biasing resistorsRB1+ and RB2+. An additional diode DB4+ together with a zener diode ZB1+are connected to the bases of the transistors Q1 and Q2.

Connected across the collectors and emitters of the driven transistorsQ1+ and Q2+ are flyback and snubber circuits including a flyback diodeDF2+, a snubber diode DS+, snubber resistors RS1+, RS2+ and snubbercapacitors CS1+, CS2+.

The negative switch assembly 36 includes identical components describedabove in connection with the positive switch assembly 34. In order todistinguish between the components of the negative switch assembly andthe components of the positive switch assembly, the plus sign in thereference designations for the components of the positive switchassembly 34 are replaced by a negative signs for the components of thenegative switch assembly 36.

The neutral switch assembly 40 includes base biasing components D1Nthrough D3N, DB1N through DB4N, RB1N, RB2N and ZB2N, power switchingcomponents including a driver transistor QDN and driven transistors Q1N,Q2N and snubber components DSN, RS1N, RS2N, CS1N and CS2N. Thesecomponents correspond to the components D1+ through D3+, DB1+ throughDB4+, RB1+, RB2+, ZB2+, QD+, Q1+, Q2+, DS+, RS1+, RS2+, CS1+, and CS2+,respectively, of the positive switch assembly 34. In addition, theneutral switch assembly 40 includes diodes DN1+, DN1-, DN1N and DN2N,snubber resistors RDN2+, RDN1-, RDN1N and RDN2N and snubber capacitorsCDN2+, CDN1-, CDN1N and CDN2N.

Current flow between the neutral terminal 32 and the phase output 38 isaccomplished in a bi-directional fashion utilizing the diodes DN1+,DN1-, DN1N and DN2N, together with the switches QDN, Q1N and Q2N. Morespecifically, current flow from the phase output 38 to the neutralterminal 32 occurs through the diode DN1+, the transistors QDN, Q1N andQ2N and the diode DN2N. Conversely, current flow from the neutralterminal 32 to the phase output 38 takes place through the diode DN1N,the transistors QDN, Q1N and Q2N and the diode DN1-.

The switches of each switch assembly 34, 36 and 40 are turned on bypositive base currents supplied over lines 64, 66 and 68, respectively.The switches are turned off by drawing current out of the bases of thesetransistors over the lines 64, 66 and 68 and through additional lines70, 72 and 74 which are coupled to the bases of the driven transistorsQ1+ and Q2+, Q1- and Q2- and Q1N and Q2N, respectively. Groundreferences for the switch assemblies 34 and 40 are provided over lines77 and 79 while a ground reference is provided for the switch assembly36 by the DC link conductor 20b. It should be understood that thecircuitry for turning the transistors of these switch assemblies on andoff is unimportant to an understanding of the present invention, and isnot described in detail herein.

The components of FIG. 4 may be packaged as three separate units asillustrated by the dashed lines of FIG. 4 which in turn leads to adesirable standardization of the units. More particularly, the diodeDN1+, resistor RDN2+ and capacitor CDN2+ may be packaged on a circuitboard 80 together with the power switches and the base biasing, snubberand flyback circuits of the switch assembly 34. The diode DN1-,capacitor CDN1- and resistor RDN1- may be packaged on a circuit board 82together with the power switches and the base biasing, snubber andflyback circuits of the negative switch assembly 36. The diodes DN1N,DN2N, the capacitors CDN1N and CDN2N and the resistors RDN1N and RDN2Nmay be packaged on a single circuit board 84 with the power switches andthe base biasing and snubber circuits of the neutral switch assembly 40.Inasmuch as the neutral switch assembly 40 does not require a flybackdiode, it can be seen that such a packaging arrangement results in thesubstantially identical number of components on each circuit board 80,82 and 84, except that the board 84 includes an extra resistor andcapacitor as compared with the boards 80 and 82. This arrangement ofcomponents allows the design of a modular circuit board so that a singleblank board or modular substrate may be configured to form a positivecircuit board, a negative circuit board, or a portion of a neutralcircuit board.

FIG. 5 illustrates the positive circuit board 80 in detail from a firstor a top side thereof while FIG. 6 illustrates the board from a secondor lower side thereof. As seen in these Figs., the board includes asubstrate 100 which is bonded to a heat exchanger 102. The heatexchanger 102 is electrically and thermally conductive and is preferablyfabricated of aluminum. The heat exchanger 102 forms a collector bus forthe switch assembly 34. As seen specifically in FIG. 7, the diodes DS+,DF2+ and DN1+ are disposed in apertures 103a-103c, respectively, throughthe substrate 100 in thermal contact with a first side of the heatexchanger 102 while the transistors QD+, Q1+ and Q2+ are disposed on inelectrical and thermal contact with a second side of the heat exchanger102. The diodes DS+ and DF2+ are directly electrically connected to thecollector bus formed by the heat exchanger 102. As seen in the explodedview from the top side of FIG. 7, a conductive pad 104 and anelectrically insulating Kapton spacer 105 are disposed between the anodeof the diode DN1+ and the heat exchanger 102 so that the anode can beconnected to other components described below. Kapton is a registeredtrademark of E.I. du Pont de Nemours and Company for flexible filmelectrical insulation.

The diodes D1+ through D3+, DB1+ through DB4+ and ZB1+, the resistorsRB1+ and RB2+ and the capacitors CS1+, CS2+ and CDN2+ are mounted on thesubstrate 100. The resistors RS1+, RS2+ and RDN2+ comprise planar etchedfoil resistors which are formed as a layer by an etching process as partof the substrate 100 and are shown in FIG. 9. A series of conductivetraces forming a trace layer 106 and a series of via holes 107 (severalof which are identified in FIG. 7) are provided as a part of thesubstrate 100. The substrate 100 further includes a main layer 101having three buses comprising an emitter bus 108, an input/output bus109 and first and second capacitor buses 110, 112. Spaces between thebuses 108, 109, 110 and 112 are occupied by electrically insulatingfiller material 114 and the portions are bonded together by a bondingagent, such as epoxy, to form the main layer 101. The filler material114 may be a commercially available product, commonly known as G10material, which conforms to military specification MIL A3949.

The trace layer 106 includes a series of traces 120, 122, 124, 126, 128,130, 132, 134 and 136 which are deposited on the main layer 101, againby an etching process. The via holes 107, the traces 120-136, the buses108, 109, 110 and 112, the heat exchanger 102, the conductive pad 104,additional conductive pads 140, 142 and electrically conductive plates144, 146 and 148 interconnect the various components on the board 80.Various junction points in the schematic of FIG. 4 are identified by anumber preceded by the letter E and are shown in FIGS. 9 and 10. Theschematic of FIG. 4 also includes reference numerals identifying theinterconnection of the components on the board by the conductive pads104, 140, 142, the heat exchanger 102, the conductive traces 120-136,the buses 108, 109, 110 and 112 and the plates 144, 146 and 148 as wellas conductive mounting posts hereinafter described.

More particularly, the trace 122 couples connection points E1 and E2 tothe anodes of the diodes D1+ and D2+ and to the cathode of the diodeD3+. The trace 124 couples the cathode of the diode D1+ to a connectionpoint E19 which is in turn coupled by a wire 160a, FIG. 6, directly tothe heat exchanger 102. A further connection point E22 is coupled to theheat exchanger 102 by a further wire 160b. The wires 160a and 160b aresoldered to the connection points E19 and E22 and are bolted to the heatexchanger 102.

The conductive trace 120, FIG. 10, connects the diodes D2+, D3+ and theresistor RB1+ to a connection point E12. The connection point E12 isalso coupled by the conductive plate 148 and a pair of conductivemounting posts 162a, 162b, FIG. 6, to the base or control electrode ofthe driver transistor QD+. The mounting posts are secured to thesubstrate 100 and are soldered to the conductive plate 148 at points 164to securely hold the emitter of the transistor QD+ in electrical contactwith the heat exchanger 102.

The conductive traces 126, 128, 130 and 134, FIG. 10, connect the diodesDB1+ through DB3+ in series between the connection point E12 and a pairof connection points E3, E4. These latter two connection points arecoupled to the line 70.

The conductive trace 134 also connects the resistor RB1+ to a pair ofconnection points E20 and E21. These connection points are coupled tothe bases of the transistors Q1+ and Q2+ by the conductive plates 144,146, FIGS. 6 and 7. The emitter of the transistor QD+ is coupled to thebases of the transistors Q1+ and Q2+ by soldering or otherwiseelectrically connecting the conductive plates 144, 146 to an emitterelectrode 169 of the transistor QD+, as seen in FIG. 6.

The conductive traces 132, 136 connect the diodes DB4+ and ZB1+ betweenthe connection point E21 and the emitter bus 108. The conductive trace136 connects the emitter bus 108 to connection points E5-E8, E13, E14and E16. In addition, a connection point E9 is coupled to the emitterbus 108 and a connector 170 is mounted on the board 80 which is in turncoupled to the phase output terminal 38. A connection point E24 is alsocoupled to the emitter bus 108.

As seen specifically in FIG. 6, emitter electrodes 176, 178 of thetransistors Q1+ and Q2+, respectively, are coupled by electricallyconductive mounting posts 180, 182, respectively, to the emitter bus108.

Referring again to FIGS. 7 and 10, the conductive pad 142 connects theanode of the transistor DS+ to the heat exchanger 102. A cathode plate183 of the diode DS+ is coupled by a pair of electrically conductivemounting posts 186a, 186b to a pair of connection points E17. Theconnection points E17 are in turn coupled to the capacitor buses 110,112. The capacitor CS1+ is soldered into via holes 190, 192 so that thecapacitor CS1+ is connected between the capacitor bus 110 and theemitter bus 108. In like fashion, the capacitor CS2+ is connectedbetween the capacitor bus 112 and the emitter bus 108 by soldering theelectrodes thereof into via holes 194, 195.

The holes 190 and 194 are plated through so that the capacitors CS1+ andCS2+ are connected by the buses 110, 112 to the etched foil resistorsRS1+ and RS2+. As seen in FIG. 9, the resistor RS1+ includes terminals196, 197 which are connected between connection points E17 and E22whereas the resistor RS2+ includes terminals 198, 199 which areconnected between the connection points E17 and E19.

The cathode of the diode DF2+ is connected by the electricallyconductive pad 140 to the heat exchanger 102. An anode terminal 200 ofthe diode DF2+ is coupled by an electrically conductive post 204, FIG.7, to the emitter bus 108.

A connector 206 comprises a part of the heat exchanger 102 and comprisesa connection point E10. In the case of the board 80, the connection 200provides a means for connecting the DC link conductor 20a thereto. Aconnection point E10 is thus formed on the heat exchanger 102.

An anode of the diode DN1+, FIGS. 7 and 10, is coupled by the conductivepad 104 and a jumper 210 to the connection point E24. A cathodeelectrode 212 of the diode DN1+ is coupled to an electrically conductivemounting post 214 which is shown in detail in FIG. 7. The connectionpoint E15 is in turn coupled to the input/output bus 109, as areconnection points E11 and E18. As seen in FIG. 10, a connector 220 iscoupled to the input/output bus 109 to allow the board 80 to beconnected to the board 84.

The capacitor CDN2+ is soldered into via holes 222 and 224 which, asseen in FIG. 9, extend through the emitter bus 108 into the etched foilresistor layer. The via holes 224 are not plated fully through the board80 so that the pins of the capacitor CDN2+ soldered therein do not makeelectrical contact with the emitter bus 108. These pins of the capacitordo, however, establish electrical contact with a terminal 228 of theetched foil resistor RDN2+ so that the capacitor CDN2+ is coupledbetween the emitter bus and such resistor.

A second terminal 230 of the resistor RDN2+ is coupled to the connectionpoint E18, which is in turn coupled by the input/output bus 109 to theconnection point E11.

Referring now to FIG. 11, the negative circuit board 82 for the negativeswitch assembly 36 is identical to the positive circuit board 80 withthe following exceptions. Elements common to FIGS. 10 and 11 areassigned like reference numerals with the exception of electricalcomponents wherein a minus sign is substituted for a plus sign, as notedpreviously. The diode DN1- is turned over as compared with the diodeDN1+ so that the anode of the diode DN1- is coupled to the connectionpoint E15. Further, the electrically insulating spacer 105 is not usedso that the cathode of the diode DN1- is coupled electrically andthermally to the heat exchanger 102.

The jumper 210 is not utilized in the negative switch assembly of FIG.11. Rather, a jumper 230 connects the electrically conductive pad 104,and hence the cathode of the diode DN1-, to a connection point E23 whichis in turn coupled to a first terminal 232 of the etched foil resistorRDN1-, seen in FIG. 9. A capacitor CDN1- includes pins which extendthrough via holes 236 to connect to a second terminal 238 of theresistor RDN1-. A second set of pins of the capacitor CDN1- extendthrough via holes 240 to establish electrical contact with theinput/output bus 109.

The capacitor CDN2+ is not used in the negative switch assembly 36. Byomitting this capacitor, the resistor RDN2+ is likewise not used.

Referring now to FIG. 12, the neutral circuit board 84 is illustrated ingreater detail. Again, elements common between FIGS. 10 and 12 areassigned like reference numerals, with the exception that electricalcomponents are identified by the suffix N instead of the plus sign. Asbefore, only the differences between the boards 80 and 84 will bedescribed in detail.

The insulating spacer 105 is not used in the board 84 so that thecathode of a diode DN1N is coupled to the heat exchanger 102. Instead,the electrically insulating spacer 150 shown in phantom in FIG. 7 isplaced between the conductive pad 140 and the heat exchanger 102 so thatthe cathode of a diode DN2N is insulated therefrom. An electricallyconductive jumper 252 connects the cathode to the connection point E18.This connection point is in turn coupled by the input/output bus 109 toa capacitor CDN1N and an anode electrode 254 of a diode DN1N via themounting post 214. The diode DN1N is turned upside down as compared withthe diode DN1+ of the board 80 so that the cathode electrode thereoffaces and is in electrical contact with the heat exchanger 102. Thecathode electrode of the diode DN1N and the heat exchanger 102 arecoupled by the conductive pad 140 and the jumper 230 to the connectionpoint E23.

The capacitor CDN1N is coupled between the connection point E15 and theresistor RDN1N. A capacitor CDN2N is coupled to the anode of the diodeDN2N by the emitter bus 108 at the connection point E16. The resistorRDN2N is connected between the capacitor CDN2N and the connection pointE18.

Referring again to FIG. 4, the boards 80, 82 and 84 are interconnectedby wires or other conductors such that the connection point E11 of theboard 80 is connected to the connection point E10 of the board 84 usingthe connectors 220 and 206 and so that the connection point E11 of theboard 82 is connected to the connection point E9 of the board 84 usingthe connectors 20 and 170. The phase output terminal 38 is coupled tothe connection point E24 of the board 80 via the connector 170 and tothe connection point E10 of the board 82 via the connector 206. The DClink conductor 20a is coupled to the connection point E10 of the board80 using the connector 206. The DC link conductor 20b is coupled to theemitter bus 108 of the board 82 using the connector 170. The neutralterminal 32 is coupled to the connection point E11 of the board 84 bythe connector 220. The lines 64, 66, 68, 70, 72, 74, 77 and 79 arecoupled to the respective boards 80, 84 using wires or other conductorswhich are soldered or otherwise electrically connected to the respectiveconnection points thereof.

If desired, the boards 80, 82 and 84 may be arranged in a stackedconfiguration with electrically insulating sheets or spacerstherebetween which may be made of Kapton or another suitable material.In such case, a threaded bar or rod (not shown) may extend throughaligned holes 270 in the boards, 80, 82 and 84 and through aligned holesin electrically insulating end caps (not shown) which are disposedadjacent the outer faces of the resulting stacked boards 80-84. Nuts(not shown) may be threaded on to the threaded bar to move the end capstoward one another to thereby force the diodes and capacitors intointimate contact with the heat exchangers 102 so that good electricalcontact is made therebetween.

As seen in FIG. 7, the heat exchanger 102 includes a passage 280 havinga plurality of cooling fins 282 disposed therein. Cooling oil or othercooling fluid enters one of two ports 284, 286 and exits the other port.When the boards 80, 82 and 84 are assembled in stacked relation, theports 284, 286 of the boards 80, 84 may be interconnected so that theheat exchangers 102 are effectively coupled in parallel to receivecooling medium.

FIGS. 8 and 9 illustrate the construction of the modular substrate 100in greater detail. The main layer 101 comprises the buses 108, 109, 110and 112 together with the filler 114 and the epoxy. A first or upperside of the main layer 101 is covered with an electrically insulatingcoating 280 which has been etched as required, for example at areas 282.The electrically conductive traces 120-136 are then formed on theinsulating layer 280 and a further insulating layer 288 is formed atopthe traces 120-136. Again, the layer 288 is etched where necessary toallow connections to the traces and to other electrical components ofthe board.

As seen in FIG. 9, a still further electrically insulating layer 290 isdeposited on a second or lower side of the first layer 101, again withportions etched where necessary. A layer of electrically resistivematerial is then deposited on the layer 290 and portions thereof areetched to form the etched foil resistors. Finally, an electricallyinsulating layer 292 is deposited over the etched foil resistors toinsulate same. The layer 292 is etched where necessary to allowconnections between the electrical components of the various layers.

As seen in the Figures, the main layer 101 occupies the majority of thethickness of the substrate 100 so that the buses 108, 109, 110 and 112and filler 114 extend substantially the entire distance between firstand second faces of the substrate 100. These portions provide mechanicalrigidity for the circuit boards 80, 82 and 84.

In the preferred embodiment, the value and rating of correspondingcomponents on the circuit boards 80, 82 and 84 are the same so that theboards may be manufactured and shipped with such components thereonabsent the jumpers 210, 230 and 252 and without the insulating pads orspacer 105 and 250. Such a board blank may then be configured for apositive switch assembly 34 by adding the insulating pad 103, the jumper210 and the capacitor CDN2+, as noted in greater detail above. The boardmight alternatively be configured as the negative switch assembly 36 byadding the jumper 230 and the capacitor CDN1- and by reversing the diodeDN1- as compared with the diode DN1+ or may be configured as a neutralswitch assembly 40 by adding the jumpers 230 and 252, the insulating pad250 and the capacitors CDN1N, CDN2N and by reversing the diode DN1N ascompared with the diode DN1+.

It can thus be seen that a modular substrate or board blank may beprovided with components mounted thereon which are common to thepositive, negative and neutral switch assemblies, together withadditional components which would allow the board to be configured asone of the positive or negative switch assemblies or as a portion of theneutral switch assembly. This desirable standardization simplifiesreplacement of boards containing one or more faulty components andobviates the need to stock separate replacement boards for each switchassembly. A significant reduction in inverter down time and inventorycost may thus be realized.

We claim:
 1. A modular inverter, comprising:at least two identicalplanar substrates; means for mounting electrical components on thesubstrates to provide at least two circuit boards; means forinterconnecting the electrical components on the circuit boards; andmeans for configuring the circuit boards to produce an alternatingcurrent output from a direct current input.
 2. The modular inverter ofclaim 1 wherein each substrate comprises a main electrically conductivebus layer, first and second layers of electrically insulating materialdisposed on first and second sides of the main layer, a series of tracesdisposed on the first layer of electrically insulating material, a thirdlayer of electrically insulating material disposed on the series oftraces, a planar resistor layer disposed on the second layer of theelectrically insulating material, and a fourth layer of electricallyinsulating material disposed on the planar resistor.
 3. The modularinverter of claim 1 wherein two circuit boards are configured to providea positive and a negative switch assembly.
 4. The modular inverter ofclaim 3 wherein the substrates of the positive and negative switchassemblies include the same number of electrical components.
 5. Themodular inverter of claim 1 wherein the electrical components mounted oneach substrate include a diode and a power transistor and wherein a heatexchanger is disposed on each substrate in thermal contact with thediode and the power transistor mounted on such substrate.
 6. The modularinverter of claim 5 wherein first, second, and third identicalsubstrates are assembled in stacked relationship, and whereinsemiconductor devices are mounted on each substrate and the substratesare configured to provide positive, negative and neutral switchassemblies.
 7. The modular inverter of claim 6 wherein some componentsfor the neutral switch assembly are mounted on the negative and thepositive circuit boards.
 8. The modular inverter of claim 7 whereinthree power diodes and three power transistors are mounted on each ofthe first, second, and third substrates.
 9. An inverter,comprising:first and second substantially identical substrates eachincluding a main electrically conductive bus layer, first and secondlayers of electrically insulating material disposed on first and secondsides of the main layer, a series of traces disposed on the first layerof electrically insulating material, a third layer of electricallyinsulating material disposed on the series of traces, a planar resistorlayer disposed on the second layer of the electrically insulatingmaterial and a fourth layer of electrically insulating material disposedon the planar resistor; a plurality of resistors and diodes on eachsubstrate interconnected by the traces; first and second electricallyconductive heat exchangers disposed on the fourth insulating layer ofthe first and second substrates, respectively; and at least one powertransistor and at least one power diode in thermal contact with eachheat exchanger and in electrical contact with the main layer bus of therespective substrate.
 10. The apparatus of claim 9 wherein three powerdiodes and three power transistors are mounted in thermal contact withsaid heat exchanger and are disposed in apertures in the substrate.